Transistor and display device

ABSTRACT

It is an object to manufacture a highly reliable display device using a thin film transistor having favorable electric characteristics and high reliability as a switching element. In a bottom gate thin film transistor including an amorphous oxide semiconductor, an oxide conductive layer having a crystal region is formed between an oxide semiconductor layer which has been dehydrated or dehydrogenated by heat treatment and each of a source electrode layer and a drain electrode layer which are formed using a metal material. Accordingly, contact resistance between the oxide semiconductor layer and each of the source electrode layer and the drain electrode layer can be reduced; thus, a thin film transistor having favorable electric characteristics and a highly reliable display device using the thin film transistor can be provided.

TECHNICAL FIELD

The present invention relates to a display device including an oxidesemiconductor.

BACKGROUND ART

In recent years, a technique for forming a thin film transistor (TFT) byusing a semiconductor thin film (having a thickness of approximatelyseveral nanometers to several hundred nanometers) formed over asubstrate having an insulating surface has attracted attention. Thinfilm transistors are applied to a wide range of electronic devices suchas ICs or electro-optical devices, and thin film transistors are rapidlydeveloped particularly as switching elements in image display devices.Various metal oxides are used for a variety of applications. Indiumoxide is a well-known material and is used as a light-transmittingelectrode material which is necessary for liquid crystal displays andthe like.

Some metal oxides have semiconductor characteristics. The examples ofsuch metal oxides having semiconductor characteristics include tungstenoxide, tin oxide, indium oxide, zinc oxide, and the like. Thin filmtransistors in which a channel formation region is formed using such ametal oxide having semiconductor characteristics are already known (forexample, see Patent Documents 1 and 2).

A thin film transistor including an amorphous oxide semiconductor hasrelatively high field-effect mobility among thin film transistorsincluding other amorphous semiconductors. Therefore, a driver circuit ofa display device or the like can also be formed using the thin filmtransistor including an amorphous oxide semiconductor.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No. 2007-96055

DISCLOSURE OF INVENTION

When a pixel portion (also referred to as a pixel circuit) and a drivercircuit portion are formed over one substrate in a display device or thelike, a transistor used for the pixel portion needs to have excellentswitching characteristics, for example, a high on/off ratio, and a thinfilm transistor used for the driver circuit portion needs to operate athigh speed.

In particular, as a display device has higher pixel density, thetransistor used for the driver circuit portion preferably operates at ahigher speed in order that time to write a display image is shortened.

An embodiment of the present invention disclosed in this specificationrelates to a transistor which solves the above-described problems and adisplay device including the transistor.

An embodiment of the present invention disclosed in this specificationis a transistor where an oxide conductive layer is included in a sourceregion and a drain region and a semiconductor layer is formed using anoxide semiconductor. In addition, the above embodiment includes adisplay device where a driver circuit portion and a display portion(also referred to as a pixel portion) which each include the transistorare formed over one substrate.

According to one embodiment of the present invention disclosed in thisspecification, a transistor includes a gate electrode layer, a gateinsulating layer over the gate electrode layer, an oxide semiconductorlayer over the gate insulating layer, an oxide conductive layer partlyoverlapping with the oxide semiconductor layer over the gate insulatinglayer, a source electrode layer and a drain electrode layer over theoxide conductive layer, and an oxide insulating layer in contact withthe oxide semiconductor layer. The oxide conductive layer includes acrystal region.

According to another embodiment of the present invention disclosed inthis specification, a transistor includes a gate electrode layer, a gateinsulating layer over the gate electrode layer, a source electrode layerand a drain electrode layer over the gate insulating layer, an oxideconductive layer over the source electrode layer and the drain electrodelayer, an oxide semiconductor layer partly overlapping with the oxideconductive layer over the gate insulating layer, and an oxide insulatinglayer in contact with the oxide semiconductor layer. The oxideconductive layer includes a crystal region.

In the above respective structures, the source electrode layer and thedrain electrode layer of the transistor can be formed using a filmincluding a metal element selected from Al, Cr, Cu, Ta, Ti, Mo, and W asits main component or an alloy film thereof. The source electrode layerand the drain electrode layer are not limited to single layerscontaining the above-described element, and stacked layers of differentfilms may also be used.

The oxide conductive layer is formed between the oxide semiconductorlayer and each of the source electrode layer and the drain electrodelayer of the transistor, whereby contact resistance can be reduced;therefore, a transistor capable of high-speed operation can be realized.The oxide conductive layer can be formed using a film of indium oxide,an indium oxide-tin oxide alloy, an indium oxide-zinc oxide alloy, zincoxide, zinc aluminum oxide, zinc aluminum oxynitride, zinc galliumoxide, or the like, which includes a crystal region.

In addition, in the above respective structures, the oxide insulatinglayer functions as a channel protective layer of the transistor. Theoxide insulating layer can be formed using silicon oxide, siliconnitride oxide, aluminum oxide, aluminum oxynitride, or the like, whichis formed with a sputtering method.

Moreover, a display device can be manufactured using an EL element, aliquid crystal element, an electrophoretic element, or the like byforming a driver circuit portion and a display portion (also referred toas a pixel portion) over one substrate using the transistor which is anembodiment of the present invention.

According to another embodiment of the present invention disclosed inthis specification, a display device includes a pixel portion and adriver circuit portion each including a transistor over one substrate,and the transistor includes a gate electrode layer over the substrate, agate insulating layer over the gate electrode layer, an oxidesemiconductor layer over the gate insulating layer, an oxide conductivelayer partly overlapping with the oxide semiconductor layer over thegate insulating layer, a source electrode layer and a drain electrodelayer over the oxide conductive layer, and an oxide insulating layer incontact with the oxide semiconductor layer. The oxide conductive layerincludes a crystal region.

According to another embodiment of the present invention disclosed inthis specification, a display device includes a pixel portion and adriver circuit portion each including a transistor over one substrate,and the transistor includes a gate electrode layer over the substrate, agate insulating layer over the gate electrode layer, a source electrodelayer and a drain electrode layer over the gate insulating layer, anoxide conductive layer over the source electrode layer and the drainelectrode layer, an oxide semiconductor layer partly overlapping withthe oxide conductive layer over the gate insulating layer, and an oxideinsulating layer in contact with the oxide semiconductor layer. Theoxide conductive layer includes a crystal region.

In a pixel portion including a plurality of transistors of the displaydevice which is an embodiment of the present invention, there is also aregion where a gate electrode of one transistor is connected to a sourcewiring or a drain wiring of another transistor. In addition, in a drivercircuit portion of the display device which is an embodiment of thepresent invention, there is a region where a gate electrode of atransistor is connected to a source wiring or a drain wiring of thetransistor.

Since a transistor is easily broken due to static electricity or thelike, a protective circuit for protecting a transistor of a pixelportion is preferably provided over the same substrate as a gate line ora source line. The protective circuit is preferably formed using anon-linear element including an oxide semiconductor layer.

An oxide conductive layer including a crystal region is formed betweenan oxide semiconductor layer and each of a source electrode layer and adrain electrode layer, whereby a transistor having favorable electriccharacteristics and high reliability can be manufactured.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1E are cross-sectional process views of a transistoraccording to an embodiment of the present invention.

FIGS. 2A to 2E are cross-sectional process views of a transistoraccording to an embodiment of the present invention.

FIGS. 3A to 3E are cross-sectional process views of a transistoraccording to an embodiment of the present invention.

FIG. 4 is a cross-sectional view of a display device according to anembodiment of the present invention.

FIG. 5 is a cross-sectional view of a display device according to anembodiment of the present invention.

FIGS. 6A1 and 6A2 are plan views and FIG. 6B is a cross-sectional viewillustrating a display device.

FIG. 7A is a plan view and FIG. 7B is a cross-sectional viewillustrating a display device.

FIGS. 8A and 8B each illustrate an electronic device.

FIGS. 9A and 9B each illustrate an electronic device.

FIG. 10 illustrates an electronic device.

FIG. 11 shows TDS spectra of moisture.

FIG. 12 shows TDS spectra of H.

FIG. 13 shows TDS spectra of O.

FIG. 14 shows TDS spectra of OH.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways without departing from the spiritand the scope of the present invention. Therefore, the present inventionis not construed as being limited to description of the embodimentsbelow. Note that in the drawings of this specification, the identicalportions or portions having a similar function are denoted by theidentical reference numerals, and description thereon may be omitted.

Embodiment 1

In this embodiment, an embodiment of a thin film transistor and amanufacturing method thereof will be described.

FIG. 1E illustrates a cross-sectional view of a thin film transistor 440having a bottom gate structure which is referred to as a channel-etchedtype.

The thin film transistor 440 includes, over a substrate 400 having aninsulating surface, a gate electrode layer 421 a, a gate insulatinglayer 402, an oxide semiconductor layer including a channel formationregion 443, a source electrode layer 445 a, and a drain electrode layer445 b. In addition, an insulating layer 427 and a protective insulatinglayer 428 are provided over the channel formation region 443, an oxideconductive layer 446 a, an oxide conductive layer 446 b, the sourceelectrode layer 445 a, and the drain electrode layer 445 b.

A first region 444 c and a second region 444 d of the oxidesemiconductor layer, which each overlap with an oxide insulating layer426, may be provided in an oxygen-excess state like the channelformation region 443 so as to have a function of reducing leak currentand parasitic capacitance.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps or the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify thepresent invention.

Here, the oxide conductive layers 446 a and 446 b are formed using amaterial with high conductivity, which includes a crystal region, andcan reduce contact resistance between the oxide semiconductor layer andeach of the source electrode layer 445 a and the drain electrode layer445 b, so that a thin film transistor capable of high-speed operationcan be realized.

A process of manufacturing the thin film transistor 440 is describedbelow with reference to FIGS. 1A to 1E.

First, after a conductive film is formed over the substrate 400 havingan insulating surface, the gate electrode layer 421 a is formed in afirst photolithography step.

Note that a resist mask used for the photolithography step may be formedwith an ink-jet method. When the resist mask is formed with an ink-jetmethod, manufacturing costs can be reduced because a photomask is notused.

As a material of the conductive film of the gate electrode layer 421 a,an element selected from Al, Cr, Ta, Ti, Mo, and W, an alloy includingthe above element, a stacked film in which any of the above-describedelements are combined, and the like can be given. Alternatively, metaloxide or the like may be used.

As the substrate 400, a glass substrate of aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass can be used, forexample. In the case where the temperature at which heat treatment isperformed later is high, it is preferable to use a glass substrate whosestrain point is greater than or equal to 730° C.

Note that, instead of the glass substrate described above, a substrateformed using an insulator, such as a ceramic substrate, a quartzsubstrate, or a sapphire substrate, may be used as the substrate 400.

Although not illustrated, an insulating layer serving as a base film maybe provided between the substrate 400 and the gate electrode layer 421a. The base film has a function of preventing diffusion of an impurityelement from the substrate 400, and can be formed using a single-layerstructure or a stacked structure of one or more of a silicon nitridefilm, a silicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

Next, the gate insulating layer 402 is formed over the gate electrodelayer 421 a.

The gate insulating layer 402 can be formed using a light-transmittinginsulating film such as a silicon oxide layer, a silicon nitride layer,a silicon oxynitride layer, or a silicon nitride oxide layer with aplasma CVD method, a sputtering method, or the like. The gate insulatinglayer 402 is not limited to the single layer of the insulating filmdescribed above, and a stacked layer of different films may also beused. For example, a silicon oxynitride film can be formed with a plasmaCVD method using silane (SiH₄), oxygen, and nitrogen as a film formationgas. The thickness of the gate insulating layer 402 is 100 nm to 500 nminclusive. In the case of a stacked structure, for example, a first gateinsulating layer having a thickness of 50 nm to 200 nm inclusive and asecond gate insulating layer having a thickness of 5 nm to 300 nm arestacked in this order.

In this embodiment, the gate insulating layer 402 is formed using asilicon oxynitride (SiON (composition ratio: N<O)) layer having athickness of 100 nm formed with a plasma CVD method.

Next, over the gate insulating layer 402, an oxide semiconductor film isformed to a thickness of 5 nm to 200 nm inclusive, preferably, 10 nm to20 nm inclusive (see FIG. 1A). The thickness is preferably as thin as 50nm or less in order that the oxide semiconductor film keeps theamorphous state even when heat treatment for dehydration ordehydrogenation is performed after the formation of the oxidesemiconductor film. Reduction in thickness can prevent the oxidesemiconductor film from being crystallized when heat treatment isperformed after the formation of the oxide semiconductor layer.

As the material of the oxide semiconductor film, a four-component metaloxide such as an In—Sn—Ga—Zn—O-based metal oxide, a three-componentmetal oxide such as an In—Ga—Zn—O-based metal oxide, an In—Sn—Zn—O-basedmetal oxide, an In—Al—Zn—O-based metal oxide, a Sn—Ga—Zn—O-based metaloxide, an Al—Ga—Zn—O-based metal oxide, and a Sn—Al—Zn—O-based metaloxide, or a two-component metal oxide such as an In—Zn—O-based metaloxide, a Sn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, aZn—Mg—O-based metal oxide, a Sn—Mg—O-based metal oxide, an In—Mg—O-basedmetal oxide, an In—O-based metal oxide, a Sn—O-based metal oxide, and aZn—O-based metal oxide can be used. The above oxide semiconductor filmmay include SiO₂.

As the oxide semiconductor film, a thin film expressed by InMO₃(ZnO)_(M) (m>0) can be used. Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co. For example, Ga, Ga and Al,Ga and Mn, or Ga and Co can be given as M. An oxide semiconductor filmwhose composition formula is represented as InMO₃ (ZnO)_(M) (m>0) whereat least Ga is included as M is referred to as the In—Ga—Zn—O-basedoxide semiconductor described above, and a thin film thereof is alsoreferred to as an In—Ga—Zn—O-based film.

In this embodiment, as the oxide semiconductor film, an In—Ga—Zn—O-basedfilm having a thickness of 15 nm is formed with a sputtering methodusing a target for forming an In—Ga—Zn—O-based oxide semiconductor.

Formation of the In—Ga—Zn—O-based film can be performed using anIn—Ga—Zn—O-based oxide semiconductor target (In₂O₃:Ga₂O₃:ZnO=1:1:1 [in amolar ratio] (that is, In:Ga:Zn=1:1:0.5 [in an atomic ratio])) under thefollowing conditions: the distance between a substrate and a target is100 mm; the pressure is 0.6 Pa; the direct-current (DC) power supply is0.5 kW; and the atmosphere is oxygen (the flow rate of oxygen is 100%).Alternatively, a target having a composition ratio of In:Ga:Zn=1:1:1 [inan atomic ratio] or In:Ga:Zn:=1:1:2 [in an atomic ratio] may be used. Inaddition, the filling factors of these targets are 90% to 100%inclusive, preferably, 95% to 99.9% inclusive. An oxide semiconductorfilm which is formed becomes dense with the use of a metal oxide targethaving a high filling factor.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power source, aDC sputtering method in which a DC power source is used, and a pulsed DCsputtering method in which a bias is applied in a pulsed manner. An RFsputtering method is mainly used in the case where an insulating layeris formed, and a DC sputtering method is mainly used in the case where ametal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

Furthermore, there are also a reactive sputtering method in which atarget substance and a sputtering gas component are chemically reactedwith each other during film formation to form a thin compound filmthereof, a bias sputtering method in which voltage is also applied to asubstrate during film formation, and the like.

Note that before the oxide semiconductor film is formed with asputtering method, dust on a surface of the gate insulating layer 402 ispreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which, without application of voltage to a target side, asurface of a substrate is modified in such a manner that an RF powersource for voltage application is used to a substrate side under anargon atmosphere and ionized argon is collided with the substrate. Notethat nitrogen, helium, oxygen, or the like may be used instead of argon.

Before the oxide semiconductor film is formed, heat treatment (at higherthan or equal to 400° C. and lower than the strain point of thesubstrate) may be performed under an atmosphere of an inert gas (e.g.,nitrogen, helium, neon, or argon) so that impurities such as hydrogenand water, which are included in the gate insulating layer 402, areremoved.

Then, the oxide semiconductor film is processed into an island-shapedoxide semiconductor layer in a second photolithography step (see FIG.1B). Further, a resist mask for forming the island-shaped oxidesemiconductor layer 404 may be formed with an ink-jet method. When theresist mask is formed with an ink-jet method, manufacturing costs can bereduced.

Next, the oxide semiconductor layer 404 is dehydrated or dehydrogenated.First heat treatment for dehydration or dehydrogenation is performed ata temperature which is higher than or equal to 400° C., preferably,higher than 425° C., and lower than the strain point of a substrateunder an atmosphere of an inert gas such as nitrogen or a rare gas(e.g., helium, neon, or argon), using an electric furnace or the like.Note that in the case of the temperature which is higher than 425° C.,the heat treatment time may be 1 hour or shorter, whereas in the case ofthe temperature which is lower than 425° C., the heat treatment time islonger than 1 hour. As another heating method, rapid thermal anneal(RTA) treatment which is performed at 650° C. for approximately 3minutes using a high-temperature inert gas or light may be performed.Since dehydration or dehydrogenation can be performed in a short timewith the RTA method, the first heat treatment can be performed even at atemperature over the strain point of a glass substrate.

Note that in this specification, heat treatment under an atmosphere ofan inert gas such as nitrogen, a rare gas, or the like is referred to asheat treatment for dehydration or dehydrogenation. In thisspecification, “dehydrogenation” does not indicate only elimination ofH₂. For convenience, elimination of H, OH, and the like is referred toas “dehydration or dehydrogenation”.

In addition, it is important not to remix water or hydrogen into theoxide semiconductor layer which is dehydrated or dehydrogenated withoutexposure to the air. In a transistor using an oxide semiconductor layerwhich is obtained in such a manner that an i-type oxide semiconductorlayer is changed into an n-type (e.g., if-type or n⁺-type) oxidesemiconductor layer, i.e. a low-resistant oxide semiconductor layer, bydehydration or dehydrogenation and then the n-type oxide semiconductorlayer is changed into an i-type oxide semiconductor layer again so as tohave high resistance, the threshold voltage of the transistor can bepositive voltage, so that the transistor shows so-called normally-offcharacteristics. It is preferable that a transistor used for a displaydevice be formed with a positive threshold voltage which is as close to0 V as possible. In an active matrix display device, the electriccharacteristics of a transistor included in a circuit are important andinfluence the performance of the display device. The threshold voltageof the transistor is particularly important. Note that if the thresholdvoltage of the transistor is negative, the transistor shows so-callednormally-on characteristics; in other words, current flows between asource electrode and a drain electrode even when the gate voltage is 0V. Accordingly, it is difficult to control the circuit including thetransistor. Even when the transistor has positive threshold voltage, thetransistor cannot perform a switching operation itself because thedriving voltage is insufficient in the case where the absolute value ofthe threshold voltage is large. In the case of an n-channel transistor,it is preferable that a channel be formed and drain current begins toflow after the positive voltage is applied as a gate voltage. Atransistor in which a channel is not formed unless the driving voltageis high and a transistor in which a channel is formed and then a draincurrent flows even when a negative voltage is applied are not suitableas the transistor used for a circuit.

An atmosphere under which the temperature at which dehydration ordehydrogenation is performed is decreased may be switched to anatmosphere different from that under which the temperature is increasedor heat treatment is performed. For example, cooling is performed byusing the same furnace in which dehydration or dehydrogenation isperformed and by filling the furnace with a high-purity oxygen gas, ahigh-purity N₂O gas, or ultra-dry air (having a dew point of lower than−40° C., preferably, lower than −60° C.) without exposure to the air.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in nitrogen or a rare gas suchas helium, neon, or argon. Alternatively, it is preferable that nitrogenor a rare gas such as helium, neon, or argon introduced into a heattreatment apparatus have purity of 6N (99.9999%) or more, preferably, 7N(99.99999%) or more.

In the case where heat treatment is performed under an atmosphere of aninert gas, an initially i-type oxide semiconductor layer is changed intoan oxygen-deficient oxide semiconductor layer by the heat treatment tobe an n-type (e.g., n⁻-type) oxide semiconductor layer, i.e. alow-resistant oxide semiconductor layer. Then, the oxide semiconductorlayer is placed in an oxygen-excess state by formation of an oxideinsulating layer which is in contact with the oxide semiconductor layerso as to be a high-resistance oxide semiconductor layer, i.e. an i-typeoxide semiconductor layer. Accordingly, it is possible to manufacture athin film transistor having favorable electric characteristics and highreliability.

In the oxide semiconductor layer which is sufficiently dehydrated ordehydrogenated under the above conditions, at least a peak at around250° C. to 300° C. of two peaks in spectra which show discharge ofmoisture is not detected with thermal desorption spectroscopy (TDS) evenwhen the temperature of the dehydrated or dehydrogenated oxidesemiconductor layer is increased to 450° C.

FIG. 11 shows results of analyzing discharge of moisture with thermaldesorption spectroscopy (TDS) in a plurality of samples each of which issubjected to heat treatment under a nitrogen atmosphere at a certaintemperature.

The thermal desorption spectroscopy is a method for detecting andidentifying, using a quadrupole mass spectrometer, a gas component whichis discharged or generated from a sample when the sample is heated andthe temperature thereof is increased in high vacuum; thus, a gas and amolecule discharged from surfaces and insides of the samples can beobserved. With the use of a thermal desorption spectroscopy apparatusmanufactured by ESCO Ltd. (product name: 1024 amu QMS), measurement wasperformed under the following conditions: the rising temperature was atapproximately 10° C./min; the pressure was 1×10⁻⁸ (Pa) at the beginningof the measurement; and the pressure was at a degree of vacuum ofapproximately 1×10⁻⁷ (Pa) during the measurement.

FIG. 11 shows a graph that compares TDS measurement results in terms ofmoisture by manufacturing the following samples in each of which anIn—Ga—Zn—O-based film having a film thickness of 50 nm is formed overthe glass substrate: a sample which is not heated (as-deposited) and asample processed at 250° C. for 1 hour, a sample processed at 350° C.for 1 hour, a sample processed at 350° C. for 10 hours, and a sampleprocessed at 450° C. for 1 hour; and a sample of the glass substratealone (not heated). The results in FIG. 11 indicate that the higher theheating temperature under a nitrogen atmosphere is, the more moisturewhich is discharged from the In—Ga—Zn—O-based film is reduced, and that,in the sample heated at 450° C., no peak at 200° C. to 350° C. isobserved in a spectrum which shows discharge of moisture.

Observed are two peaks of spectra which show discharge of moisture at ahigh temperature of 200° C. or more from the In—Ga—Zn—O-based film: afirst peak which appears at a temperature between 200° C. and 250° C.;and a second peak which appears at a temperature between 250° C. and350° C. The first peak which appears at a temperature between 200° C.and 250° C. is not clear in the samples other than the sample heated at250° C. In the as-deposited sample, this is because two peaks overlapwith each other and thus there is apparently a spectrum having one peak.Further, in the samples which are subjected to heat treatment at 350°C., this is because moisture is somewhat discharged and thus the firstpeak is substantially disappeared. These can also be confirmed from asymmetric property of a peak position of each spectrum and the fact thatthe peak position of each spectrum is shifted to a higher temperatureside.

In addition, the vertical axis of the graph of FIG. 11 represents anarbitrary unit, where existence of discharged gas is relatively seen.These shapes of the spectra each show similar change with respect toheat treatment regardless of the area or volume of an object to bemeasured. Accordingly, the spectra can be effectively used asobservation of a process monitor or a means for failure analysis. Inother words, existence of a peak in a temperature range of 200° C. to350° C. is examined, whereby it is possible to confirm a record thatindicates whether an appropriate process has been conducted or not.

Note that even in the case where the sample which is subjected to heattreatment at 450° C. under a nitrogen atmosphere is left at roomtemperature under an air atmosphere approximately for one week,discharge of moisture at 200° C. or more was not observed. Thus, it isfound that by performing the heat treatment, the In—Ga—Zn—O-based filmbecomes stable.

Further, TDS measurement was performed using samples which have beensubjected to a heating step under the same conditions as those of thesamples in each of which discharge of moisture has been measured inorder to measure H, O, OH, H₂, O₂, N, N₂, and Ar in addition to H₂O. Itwas possible that spectra which show discharge of H, O, and OH from someof the samples be clearly observed. FIG. 12 shows TDS spectra of H. FIG.13 shows TDS spectra of O. FIG. 14 shows TDS spectra of OH. Themeasurement result of a glass substrate alone (not heated) is added toTDS spectra of moisture, H, O, and OH for comparison. Note that theoxygen concentration under a nitrogen atmosphere in the above heatconditions is 20 ppm or less.

The spectra which show discharge of H, O, and OH tend to be similar tothe spectra which show discharge of moisture and indicate that, in thesample heated at 450° C., no peak is observed which shows each dischargecomponent that appears around 250° C. to 300° C.

The above results indicate that, by performance of the heat treatment ofthe In—Ga—Zn—O-based film, moisture, H, O, and OH are discharged. Sincedischarge of H, O, and OH tends to be performed in a state similar tothat of moisture, it can be said that most of the discharge of H, O, andOH is derived from a water molecule.

Here, in this embodiment, the substrate is introduced into an electricfurnace, which is one of heat treatment apparatuses, and the heattreatment of the oxide semiconductor layer is performed under a nitrogenatmosphere. Then, the oxide semiconductor layer is not exposed to air,which prevents the oxide semiconductor layer from remixing water orhydrogen, so that an oxide semiconductor layer is obtained. In addition,slow cooling is performed in a nitrogen atmosphere in one furnace fromthe heating temperature T at which the oxide semiconductor layers aredehydrated or dehydrogenated to a temperature low enough to preventwater from coming in, specifically to a temperature more than 100° C.lower than the heating temperature T. Without limitation to a nitrogenatmosphere, dehydration or dehydrogenation is performed in a rare gasatmosphere such as helium, neon, or argon.

The oxide semiconductor layer is partly crystallized in some cases,depending on a condition of the first heat treatment or a material ofthe oxide semiconductor layer. After the first heat treatment, the oxidesemiconductor layer 404 which is changed into an oxygen-deficient oxidesemiconductor layer to be a low-resistant oxide semiconductor layer isobtained (see FIG. 1B). The carrier concentration of the oxidesemiconductor layer is higher after the first heat treatment than thatof the oxide semiconductor film just after the film formation,accordingly; it is preferable that the oxide semiconductor layer 404have a carrier concentration of 1×10¹⁸/cm³ or more. Note that the oxidesemiconductor layer is preferably amorphous but may be partlycrystallized. Note that in this specification, even in the state wherethe oxide semiconductor layer is partly crystallized, it is referred toas an “amorphous” state.

Further, the gate electrode layer 421 a is crystallized to be amicrocrystalline film or a polycrystalline film in some cases, dependingon a condition of the first heat treatment or a material of the gateelectrode layer 421 a. For example, in the case where a film of anindium oxide-tin oxide alloy is used as the gate electrode layer 421 a,the film is crystallized by the first heat treatment at 450° C. for 1hour.

Alternatively, the first heat treatment may be performed on the oxidesemiconductor film before it is processed into the island-shaped oxidesemiconductor layer. In that case, after the first heat treatment, thesubstrate is taken out from the heat treatment apparatus, and then thesecond photolithography step is performed.

Next, an oxide insulating film is formed over the gate insulating layer402 and the oxide semiconductor layer 404 with a sputtering method.Then, a resist mask is formed in a third photolithography step, and theoxide insulating layers 426 are formed by selective etching. After that,a step of removing the resist mask may be performed. At this phase, theregions are formed that overlap with the oxide insulating layers 426which cover the peripheral portion and side surface of the oxidesemiconductor layer (the first region 444 c and the second region 444 dof the oxide semiconductor layer), whereby leakage current and parasiticcapacitance can be reduced (see FIG. 1E).

The oxide insulating layer 426 can be formed with a thickness of atleast 1 nm with a method with which impurities such as water or hydrogenare not mixed into the above oxide insulating layer, as appropriate. Inthis embodiment, the oxide insulating layer 426 is formed using asilicon oxide film which is formed with a sputtering method.

The substrate temperature at the time of film formation may be from roomtemperature to 300° C. inclusive and, in this embodiment, is 100° C. Thesilicon oxide film can be formed with a sputtering method under anatmosphere of a rare gas (typically, argon), an oxygen atmosphere, or amixed atmosphere containing a rare gas (typically, argon) and oxygen.

Moreover, a silicon oxide target or a silicon target can be used as atarget. For example, with the use of a silicon target, a silicon oxidefilm can be formed with a sputtering method under an atmosphere ofoxygen and a rare gas. The oxide insulating layer which is formed incontact with the oxide semiconductor layer whose resistance is reducedis formed using an inorganic insulating film that does not containimpurities such as moisture, a hydrogen ion, OH— and blocks entry ofsuch impurities from the outside. Typically, a silicon oxide film, asilicon nitride oxide film, an aluminum oxide film, an aluminumoxynitride film, or the like can be used.

In this embodiment, the film formation is performed with a pulsed DCsputtering method using a columnar polycrystalline silicon target towhich boron is added (the resistivity is 0.01 Ωcm and the purity is 6N),in which the distance between substrate and target (T-S distance) is 89mm, the pressure is 0.4 Pa, the direct-current (DC) power is 6 kW, andthe atmosphere is oxygen (the oxygen flow rate is 100%). The filmthickness thereof is 300 nm.

Next, an oxide conductive film and a metal film are stacked over thegate insulating layer 402, the oxide insulating layers 426, and theoxide semiconductor layer 404. With a sputtering method, film formationof the stacked layer of the oxide conductive film and the metal film canbe performed continuously without exposure to air.

As the material of the oxide conductive film, for example, any of thefollowing conductive metal oxide materials can be employed: anIn—Sn—O-based metal oxide; an In—Sn—Zn—O-based metal oxide; anIn—Al—Zn—O-based metal oxide; a Sn—Ga—Zn—O-based metal oxide; anAl—Ga—Zn—O-based metal oxide; a Sn—Al—Zn—O-based metal oxide; anIn—Zn—O-based metal oxide; a Sn—Zn—O-based metal oxide; an Al—Zn—O-basedmetal oxide; an In—O-based metal oxide; a Sn—O-based metal oxide; or aZn—O-based metal oxide. The thickness of the oxide conductive film isselected as appropriate in the range of 50 nm to 300 nm inclusive. Inthis embodiment, a zinc oxide film is used.

As a material of the metal film, an element selected from Ti, Mo, W, Al,Cr, Cu, and Ta, an alloy containing any of these elements as acomponent, or the like is used. The above metal film is not limited to asingle layer and a stacked layer of different films can be used. In thisembodiment, a three-layer-stacked film in which a molybdenum film, analuminum film, and a molybdenum film are stacked is used.

Next, a resist mask is formed in a fourth photolithography step, and themetal film is selectively etched to form the source electrode layer 445a and the drain electrode layer 445 b. After that, the resist mask isremoved. Note that an alkaline solution is used as a resist stripper toremove the resist mask. In the case where the resist stripper is used,the zinc oxide film is also selectively etched using the sourceelectrode layer 445 a and the drain electrode layer 445 b as masks.

In this manner, the oxide conductive layer 446 a is formed under and incontact with the source electrode layer 445 a, and the oxide conductivelayer 446 b is formed under and in contact with the drain electrodelayer 445 b (see FIG. 1D).

By providing the oxide conductive layer 446 a between the sourceelectrode layer 445 a and the oxide semiconductor layer, contactresistance can be reduced, which leads to resistance reduction, so thata thin film transistor capable of high-speed operation can be realized.The oxide conductive layer 446 a provided between the source electrodelayer 445 a and the oxide semiconductor layer functions as a sourceregion, and the oxide conductive layer 446 b provided between the drainelectrode layer 445 b and the oxide semiconductor layer functions as adrain region, which are effective in improvement the frequencycharacteristics in the case where, for example, a peripheral circuit(driver circuit) is formed over one substrate.

In the case where a molybdenum film and the oxide semiconductor layerare directly in contact with each other, the contact resistance isincreased. This is because molybdenum is less likely to be oxidized ascompared to titanium and thus extracts a small amount of oxygen from theoxide semiconductor layer, which does not allow the interface betweenthe molybdenum and the oxide semiconductor layer to be n-type oxidesemiconductor. However, even in that case, by providing the oxideconductive layer between the oxide semiconductor layer and each of thesource electrode layer and the drain electrode layer, the contactresistance can be reduced.

The etching rate is different between the oxide semiconductor layer andthe oxide conductive layer, and therefore, the oxide conductive layerwhich is on and in contact with the oxide semiconductor layer can beremoved by controlling the time of period.

After the metal film is selectively etched, the resist masks may beremoved by an oxygen ashing treatment to leave the zinc oxide film, andthen, the zinc oxide film may be selectively etched with the sourceelectrode layer 445 a and the drain electrode layer 445 b as masks.

The resist mask for forming the source electrode layer 425 a and thedrain electrode layer 425 b may be formed with an ink-jet method.

Next, the insulating layer 427 is formed over the oxide insulatinglayers 426, the source electrode layer 445 a, the drain electrode layer445 b and oxide semiconductor layer 404. As the insulating layer 427, asilicon oxide film, a silicon nitride oxide film, an aluminum oxidefilm, an aluminum oxynitride film, or the like is used. In thisembodiment, a silicon oxide film is formed as the insulating layer 427with an RF sputtering method.

At this phase, second heat treatment is performed under an atmosphere ofan inert gas such as nitrogen at a temperature of greater than or equalto 200° C. and less than or equal to 400° C., preferably, greater thanor equal to 250° C. and less than or equal to 350° C. For example, heattreatment is performed under a nitrogen atmosphere at 250° C. for 1hour.

In the second heat treatment, part of the insulating layer 427 which isan oxide and the oxide semiconductor layer 404 are heated while being incontact with each other. Therefore, in the oxide semiconductor layer 404the resistance of which has been reduced through the first heattreatment, oxygen is supplied from the insulating layer 427 to make theoxide semiconductor layer 404 into an oxygen-excess state, whereby theoxide semiconductor layer 404 has high resistance (i-type conductivity).

Although the second heat treatment is performed after the formation ofthe silicon oxide film in this embodiment, the second heat treatment canbe performed anytime after the formation of the silicon oxide film andthe timing of the second heat treatment is not limited to just after theformation of the silicon oxide film.

In addition, in the case where the source electrode layer 445 a and thedrain electrode layer 445 b are formed using a heat-resistant material,a step in which the conditions of the first heat treatment are used canbe performed at the timing of the second heat treatment. In this case,the heat treatment can be performed only once after the formation of thesilicon oxide film.

In the second heat treatment, the oxide conductive layers 446 a and 446b are crystallized to each have a crystal region as long as acrystallization inhibitor such as silicon oxide is not contained in theoxide conductive layers 446 a and 446 b. For example, the oxideconductive layers tend to be crystallized into columnar-like crystalswhen zinc oxide or the like is used, and tend to be crystallized in amicrocrystalline state when an indium oxide-tin oxide alloy is used.Consequently, realized is reduction of contact resistance between theoxide semiconductor layer and each of the source electrode layer 445 aand the drain electrode layer 445 b as well as improvement inconductivity. On the other hand, the In—Ga—Zn—O-based oxidesemiconductor layer which is used in this embodiment is not crystallizedeven through the second heat treatment and thus an amorphous statethereof is maintained.

Next, the protective insulating layer 428 is formed over the insulatinglayer 427 (see FIG. 1E). As the protective insulating layer 428, asilicon nitride film, a silicon nitride oxide film, an aluminum nitridefilm, or the like can be used. In this embodiment, a silicon nitridefilm is formed as the protective insulating layer 428 with an RFsputtering method.

Through the steps described above, the thin film transistor 440 can bemanufactured in which the oxide conductive layer having a crystal regionis formed between the oxide semiconductor layer and each of the sourceelectrode layer and the drain electrode layer.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 2

In this embodiment will be described an example of a bottom contact thinfilm transistor and a manufacturing process thereof, which is differentfrom that of Embodiment 1, with reference to FIGS. 2A to 2E. FIGS. 2A to2E are the same as FIGS. 1A to 1E except that there is a difference inpart of the process. Therefore, the same portions are denoted by thesame reference numerals, and detailed description of the same portionsis omitted.

A thin film transistor 470 illustrated in FIG. 2E is a bottom gatestructure which is referred to as a bottom contact type.

The thin film transistor 470 includes, over the substrate 400 having aninsulating surface, the gate electrode layer 421 a, the gate insulatinglayer 402, the source electrode layer 445 a, the drain electrode layer445 b, the oxide conductive layer 446 a, the oxide conductive layer 446b, and the oxide semiconductor layer including the channel formationregion 443. In addition, the insulating layer 427 and the protectiveinsulating layer 428 are provided over the channel formation region 443,the source electrode layer 445 a, and the drain electrode layer 445 b.

Here, the oxide conductive layers 446 a and 446 b are formed using amaterial with high conductivity, which includes a crystal region, andcan reduce contact resistance between the oxide semiconductor layer andeach of the source electrode layer 445 a and the drain electrode layer445 b, so that a thin film transistor capable of high-speed operationcan be realized.

A process of manufacturing the thin film transistor 470 is describedbelow with reference to FIGS. 2A to 2E.

The gate electrode layer 421 a and the gate insulating layer 402 areformed in accordance with Embodiment 1.

Next, a metal film and an oxide conductive film are stacked over thegate insulating layer 402 (see FIG. 2A). At this time, with a sputteringmethod, film formation of the stacked layer of the metal film and theoxide conductive film can be performed continuously without exposure toair. In this embodiment, a three-layer-stacked film in which amolybdenum film, an aluminum film, and a molybdenum film are stacked isused for the metal film, and a zinc oxide film is used for the oxideconductive film.

Next, a resist mask is formed in a photolithography step, and the metalfilm and the zinc oxide film are selectively etched to form the sourceelectrode layer 445 a, the drain electrode layer 445 b, the oxideconductive layer 446 a, and the oxide conductive layer 446 b (see FIG.2B). Here, an alkaline solution is used as a resist stripper to removethe resist mask. Since the zinc oxide film is also etched in some cases,it is preferable to remove the resist mask by oxygen ashing in order toprevent the zinc oxide film from being reduced in thickness.

Next, an oxide semiconductor film is formed in a manner similar to thatof Embodiment 1, and the oxide semiconductor layer 404 is formed througha photolithography step and an etching step (see FIG. 2C).

Here, the oxide semiconductor layer is dehydrated or dehydrogenated inaccordance with the method of the first heat treatment described inEmbodiment 1.

Note that before the oxide semiconductor film is formed, heat treatment(at higher than or equal to 400° C. and lower than the strain point ofthe substrate) may be performed under an atmosphere of an inert gas(e.g., nitrogen, helium, neon, or argon) so that impurities such ashydrogen and water, which are included in the gate insulating layer 402,are removed.

Next, the insulating layer 427 is formed over the source electrode layer445 a, the drain electrode layer 445 b, and the oxide semiconductorlayer 404. As the insulating layer 427, a silicon oxide film, a siliconnitride oxide film, an aluminum oxide film, an aluminum oxynitride film,or the like is used. In this embodiment, a silicon oxide film is formedas the insulating layer 427 with an RF sputtering method.

Here, heat treatment is performed in accordance with the method of thesecond heat treatment described in Embodiment 1.

Although the second heat treatment is performed after the formation ofthe silicon oxide film in this embodiment, the second heat treatment canbe performed anytime after the formation of the silicon oxide film andthe timing of the second heat treatment is not limited to just after theformation of the silicon oxide film.

In addition, in the case where the source electrode layer 445 a and thedrain electrode layer 445 b are formed using a heat-resistant material,a step in which the conditions of the first heat treatment are used canbe performed at the timing of the second heat treatment.

Through any one of the heat treatment processes up to here, the oxideconductive layers 446 a and 446 b are crystallized to each have acrystal region as long as a crystallization inhibitor such as siliconoxide is not contained in the oxide conductive layers 446 a and 446 b.Needless to say, the heat treatment process may be performed pluraltimes. On the other hand, the oxide semiconductor layer is notcrystallized even through the heat treatment which is performed pluraltimes and thus an amorphous state thereof is maintained.

Next, the protective insulating layer 428 is formed over the insulatinglayer 427 (see FIG. 2E).

Through the steps described above, the thin film transistor 470 can bemanufactured in which the oxide conductive layer having a crystal regionis formed between the oxide semiconductor layer and each of the sourceelectrode layer and the drain electrode layer.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 3

In this embodiment will be described an example of a bottom contact thinfilm transistor and a manufacturing process thereof, which is differentfrom those of Embodiments 1 and 2, with reference to FIGS. 3A to 3E.FIGS. 3A to 3E are the same as FIGS. 1A to 1E except that there is adifference in part of the process. Therefore, the same portions aredenoted by the same reference numerals, and detailed description of thesame portions is omitted.

FIG. 3E illustrates a cross-sectional view of a thin film transistor 480having a bottom gate structure which is referred to as achannel-protected type.

The thin film transistor 480 includes, over the substrate 400 having aninsulating surface, the gate electrode layer 421 a, the gate insulatinglayer 402, an oxide semiconductor layer, the oxide conductive layer 446a, the source electrode layer 445 a, and the drain electrode layer 445b. Here, the oxide semiconductor layer includes the channel formationregion 443, a high-resistance source region 424 a, and a high-resistancedrain region 424 b. In addition, an oxide insulating layer 426 a whichfunctions as a channel protective layer is provided in contact withchannel formation region 443. Further, the protective insulating layer428 is provided over the source electrode layer 445 a, the drainelectrode layer 445 b, the channel protective layer 426 a, and the oxidesemiconductor layer 404.

A first region 424 c and a second region 424 d of the oxidesemiconductor layer, which each overlap with an oxide insulating layer426 b, are provided in an oxygen-excess state like the channel formationregion 443, and may have a function of reducing leak current andparasitic capacitance.

A third region 424 e of the oxide semiconductor layer, which is incontact with the protective insulating layer 428, is provided betweenthe channel formation region 443 and the high-resistance source region424 a. A fourth region 424 f of the oxide semiconductor layer, which isin contact with the protective insulating layer 428, is provided betweenthe channel formation region 443 and the high-resistance drain region424 b.

The oxide conductive layers 446 a and 446 b are formed using a materialwith high conductivity, which includes a crystal region, and can reducecontact resistance between the oxide semiconductor layer and each of thesource electrode layer 445 a and the drain electrode layer 445 b, sothat a thin film transistor capable of high-speed operation can berealized.

The third region 424 e and the fourth region 424 f of the oxidesemiconductor layer are a high-resistance source region (also referredto as an HRS region) which is an oxygen-deficient region and ahigh-resistance drain region (also referred to as an HRD region) whichis an oxygen-deficient region, respectively. Specifically, the carrierconcentration of the high-resistance drain region is greater than orequal to 1×10¹⁸/cm³ and is at least higher than the carrierconcentration of the channel formation region (less than 1×10¹⁸/cm³).

Note that the carrier concentration in this embodiment is obtained byHall effect measurement at room temperature. When the widths of thethird region and fourth region in the channel length direction arelarge, an off-current of the thin film transistor can be reduced. Incontrast, when the widths of the third region and fourth region in thechannel length direction are small, the operation speed of the thin filmtransistor can be increased.

In a channel-protected thin film transistor, although the width of anoxide insulating layer which functions as a channel protective layer isreduced so that a substantial channel length L is easily shortened, ashort circuit might be caused when a source electrode layer and a drainelectrode layer are provided over the oxide insulating layer. Therefore,the source electrode layer 445 a and the drain electrode layer 445 b areprovided so that end portions thereof are apart from the oxideinsulating layer.

In FIG. 3E, a region of the oxide semiconductor layer under the oxideinsulating layer 426 a which functions as a channel protective layer isreferred to as a channel formation region. Therefore, the channel lengthL of the thin film transistor 480 is equal to the width of the oxideinsulating layer 426 a in the channel length direction, and, in thecross-sectional view of FIG. 3E, corresponds to a length of the base ofthe trapezoidal oxide insulating layer 426 a.

A process of manufacturing the thin film transistor 480 is describedbelow with reference to FIGS. 3A to 3E.

The gate electrode layer 421 a and the gate insulating layer 402 areformed in accordance with Embodiment 1 (see FIG. 3A).

Next, an oxide semiconductor film is formed in a manner similar to thatof Embodiment 1, and the island-shaped oxide semiconductor layer 404 isformed through a photolithography step and an etching step (see FIG.3B).

Note that before the oxide semiconductor film is formed, heat treatment(at higher than or equal to 400° C. and lower than the strain point ofthe substrate) may be performed under an atmosphere of an inert gas(e.g., nitrogen, helium, neon, or argon) so that impurities such ashydrogen and water, which are included in the gate insulating layer 402,are removed.

Here, the oxide semiconductor layer is dehydrated or dehydrogenated inaccordance with the method of the first heat treatment described inEmbodiment 1.

Alternatively, the first heat treatment may be performed on the oxidesemiconductor film before it is processed into the island-shaped oxidesemiconductor layer.

Next, an oxide insulating film is formed over the gate insulating layer402 and the oxide semiconductor layer 404 with a sputtering method in amanner similar to that of Embodiment 1. Then, resist mask is formed in aphotolithography step, and the oxide insulating layers 426 a and 426 bare formed by selective etching. After that, the resist mask may beremoved. Here, a region of the oxide semiconductor layer under the oxideinsulating layer 426 a serves as a channel formation region (see FIG.3C).

Next, an oxide conductive film and a metal film are stacked over theoxide insulating layer 426 a, the oxide insulating layers 426 b, and theoxide semiconductor layer and are partly etched to form the sourceelectrode layer 445 a, the drain electrode layer 445 b, the oxideconductive layer 446 a, and the oxide conductive layer 446 b. With asputtering method, film formation of the stacked layer of the oxideconductive film and the metal film can be performed continuously withoutexposure to air. In this embodiment, a three-layer-stacked film in whicha molybdenum film, an aluminum film, and a molybdenum film are stackedis used for the metal film, and a zinc oxide film is used for the oxideconductive film (see FIG. 3D).

Next, second heat treatment is performed under an atmosphere of an inertgas such as nitrogen at a temperature of greater than or equal to 200°C. and less than or equal to 400° C., preferably, greater than or equalto 250° C. and less than or equal to 350° C. For example, heat treatmentis performed under a nitrogen atmosphere at 250° C. for 1 hour.

In the second heat treatment, part of the oxide semiconductor layer 404is heated while being in contact with the oxide insulating layers 426 aand 426 b. Therefore, in the oxide semiconductor layer 404 theresistance of which has been reduced through the first heat treatment,oxygen is supplied from the oxide insulating layers 426 a and 426 b tomake the oxide semiconductor layer 404 into an oxygen-excess state,whereby the oxide semiconductor layer 404 has high resistance (i-typeconductivity).

On the other hand, since part of the oxide semiconductor layer 404,which does not overlap with the oxide insulating layers 426 a and 426 b,is heated while being exposed, the third region 424 e and fourth region424 f the resistance of which is maintained or further reduced can beformed.

Note that although the second heat treatment is performed after theformation of the oxide insulating layers 426 a and 426 b in thisembodiment, the second heat treatment can be performed anytime after theformation of the oxide insulating layers 426 a and 426 b and the timingof the second heat treatment is not limited to just after the formationof the oxide insulating layers 426 a and 426 b.

Through the second heat treatment, the oxide conductive layers 446 a and446 b are crystallized to each have a crystal region as long as acrystallization inhibitor such as silicon oxide is not contained in theoxide conductive layers 446 a and 446 b. On the other hand, the oxidesemiconductor layer is not crystallized even through the second heattreatment and thus an amorphous state thereof is maintained.

Next, the protective insulating layer 428 is formed over the oxidesemiconductor layer 404, the oxide insulating layer 426 a, the oxideinsulating layers 426 b, the source electrode layer 445 a, and the drainelectrode layer 445 b (see FIG. 3E). As the insulating layer 428, asilicon nitride film, a silicon nitride oxide film, an aluminum nitridefilm, or the like is used. In this embodiment, a silicon nitride film isformed as the protective insulating layer 428 with an RF sputteringmethod.

Through the steps described above, the thin film transistor 480 can bemanufactured in which the oxide conductive layer having a crystal regionis formed between the oxide semiconductor layer and each of the sourceelectrode layer and the drain electrode layer.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 4

In this embodiment will be described an example in which an activematrix liquid crystal display device or light-emitting device ismanufactured over one substrate using the thin film transistor describedin any of Embodiments 1, 2, and 3.

FIG. 4 illustrates an example of a cross-sectional structure of a liquidcrystal display device using an active matrix substrate.

While in Embodiments 1, 2, and 3, modes of a thin film transistor areillustrated in the cross-sectional views, in this embodiment, astructure in which a driver circuit portion and a pixel portion areincluded over one substrate is described with a view illustrating thefollowing: a thin film transistor 450 for the driver circuit portion; athin film transistor 460 for the pixel portion; a gate wiring contactportion; a storage capacitor; a gate wiring, a source wiring, and anintersection thereof; a pixel electrode; and the like. The storagecapacitor, the gate wiring, and the source wiring can be formed in thesame manufacturing steps as the thin film transistors shown inEmbodiments 1 and 2 and can be manufactured without an increase in thenumber of photomasks and an increase in the number of steps.

In FIG. 4, the thin film transistor 450 is a thin film transistorprovided in the driver circuit portion and the thin film transistor 460electrically connected to a pixel electrode layer 457 a is a thin filmtransistor provided in the pixel portion.

In this embodiment, the thin film transistor 460 formed over thesubstrate 400 has the same structure as the thin film transistors inEmbodiments 1, 2, and 3. Here, a channel-etched thin film transistor isshown as an example.

A capacitor wiring layer 430 which is formed using the same material inthe same step as a gate electrode layer of the thin film transistor 460overlaps with a capacitor electrode layer 431, with a gate insulatinglayer 402 serving as a dielectric interposed therebetween; thus, astorage capacitor is formed. Note that the capacitor electrode layer 431is formed using the same material in the same step as an electrode layerand an oxide conductive layer which are provided in a source region or adrain region of the thin film transistor 460.

Note that the storage capacitor is provided below the pixel electrodelayer 457 a. Although not illustrated, the capacitor electrode layer 431is electrically connected to the pixel electrode layer 457 a.

This embodiment shows the example in which the storage capacitor isformed using the capacitor electrode layer 431 and the capacitor wiringlayer 430; however, there is no particular limitation on the structureof the storage capacitor. For example, the storage capacitor may beformed in such a manner that, without provision of a capacitor wiringlayer, a pixel electrode layer overlaps with a gate wiring in anadjacent pixel with a planarization insulating layer, a protectiveinsulating layer, and a gate insulating layer interposed therebetween.

A plurality of gate wirings, source wirings, and capacitor wiring layersare provided in accordance with the pixel density. In a terminalportion, a plurality of first terminal electrodes at the same potentialas the gate wiring, a plurality of second terminal electrodes at thesame potential as the source wiring, a plurality of third terminalelectrodes at the same potential as the capacitor wiring layer, and thelike are arranged. The number of each of the terminals to be providedmay be any number, and the number of each terminal may be determined asappropriate.

In the gate wiring contact portion, a gate electrode layer 421 b can beformed using a low resistance metal material. The gate electrode layer421 b is electrically connected to the gate wiring through a contacthole that reaches the gate wiring.

The heat treatment for dehydration or dehydrogenation of the oxidesemiconductor layer may be performed: after the oxide semiconductorlayer is formed; after an oxide conductive layer is stacked over theoxide semiconductor layer; or after a passivation film is formed over asource electrode and a drain electrode.

A gate electrode layer of the thin film transistor 450 in the drivercircuit portion may be electrically connected to a conductive layer 417provided above the oxide semiconductor layer.

Further, in the wiring intersection, in order to reduce the parasiticcapacitance as illustrated in FIG. 4, the gate insulating layer 402 andthe oxide insulating layer 426 are sacked between a gate wiring layer421 c and a source wiring layer 422.

When an active matrix liquid crystal display device is manufactured, anactive matrix substrate and a counter substrate provided with a counterelectrode are fixed with a liquid crystal layer interposed therebetween.Similarly, with a plurality of microcapsules each including firstparticles having a positive charge and second particles having anegative charge disposed between two electrodes, an active matrixelectrophoretic display device can be manufactured. A common electrodeelectrically connected to the counter electrode on the counter substrateis provided over the active matrix substrate, and a fourth terminalelectrode electrically connected to the common electrode is provided inthe terminal portion. The fourth terminal electrode is used for settingthe common electrode to a fixed potential such as GND or 0 V The fourthterminal electrode can be formed using the same material as the pixelelectrode layer 457 a.

When the same material is used for the gate electrode, the sourceelectrode, the drain electrode, the pixel electrode, another electrodelayer, and another wiring layer, a common sputtering target and a commonmanufacturing apparatus can be used, and thus the material costs andcosts of an etchant (or an etching gas) used for etching can be reduced.As a result, manufacturing costs can be reduced.

When a photosensitive resin material is used for a planarizationinsulating layer 456 in the structure of FIG. 4, the step for forming aresist mask can be omitted.

In addition, FIG. 5 illustrates a cross-sectional view in a state of asubstrate of an active matrix light-emitting device before an EL layeris formed over a first electrode (a pixel electrode).

In FIG. 5, a channel-etched thin film transistor is illustrated;however, the thin film transistor having a structure similar to thosedescribed in Embodiments 2 and 3 can also be used. Moreover, the activematrix light-emitting device illustrated in FIG. 5 can have a structuresimilar to that of the above liquid crystal display device except forthe structure of a pixel portion which will be shown below.

After an insulating layer 427 is formed, a color filter layer 453 isformed. The colors of the color filter layer are red, green, and blue.The color filter layers are sequentially formed in the specificpositions with a printing method, an ink-jet method, a photolithographytechnique, an etching method, or the like. By providing the color filterlayer 453 on the substrate 400 side, alignment of the color filter layer453 and a light-emitting region of a light-emitting element can beperformed without depending on the alignment accuracy of the sealingsubstrate.

Next, an overcoat layer 458 which covers the color filter layer 453 isformed. The overcoat layer 458 is formed using a light-transmittingresin.

Here, an example in which full-color display is performed using colorfilter layers of three colors of red, green, and blue; however, thecolor display is not particularly limited thereto. A color of cyan,magenta, yellow, or white may be used.

Next, a protective insulating layer 428 which covers the overcoat layer458 and the insulating layer 427 is formed. For the protectiveinsulating layer 428, an inorganic insulating film such as a siliconnitride film, an aluminum nitride film, a silicon nitride oxide film, oran aluminum oxynitride film is used.

Next, in a photolithography step and an etching step, a contact holethat reaches the connection electrode layer 452 is formed in theprotective insulating layer 428 and the insulating layer 427. Inaddition, in this photolithography step and etching step, the protectiveinsulating layer 428 and the insulating layer 427 in a terminal portionare selectively etched to expose part of a terminal electrode. Further,in order to connect a second electrode of a light-emitting elementformed later to a common potential line, a contact hole that reaches thecommon potential line is also formed.

Next, a transparent conductive film is formed, and a photolithographystep and an etching step are performed thereon, so that a firstelectrode 457 b which is electrically connected to the connectionelectrode layer 452 is formed.

Next, a partition wall 459 is formed to cover the periphery of the firstelectrode 457 b. The partition wall 459 is formed using an organic resinfilm of polyimide, acrylic, polyamide, epoxy, or the like, an inorganicinsulating film, or organic polysiloxane. The partition wall 459 can beformed using an organic insulating material or an inorganic insulatingmaterial. It is preferable that the partition wall 459 be formed to havean opening over the first electrode 457 b so that a sidewall is formedas an inclined surface with curvature. Such an opening can be easilyformed using a photosensitive resin material.

Through the steps described above, the state of the substrateillustrated in FIG. 5 can be obtained. Further, an EL layer is formedover the first electrode 457 b and the second electrode is formed overthe EL layer, whereby a light-emitting element is formed. The secondelectrode is electrically connected to the common potential line.

The conductive layer 417 may be provided over the oxide semiconductorlayer of the thin film transistor 450 of the driver circuit portion ineach of FIG. 4 and FIG. 5. The conductive layer 417 can be formed usingthe same material in the same step as the pixel electrode layer 457 a orthe first electrode 457 b.

The conductive layer 417 is provided so as to overlap with the channelformation region 443 of the oxide semiconductor layer, whereby theamount of change over time in threshold voltage of the thin filmtransistor 450 can be reduced. The conductive layer 417 has a potentialwhich is the same as that of the gate electrode layer 421 a, and canfunction as a second gate electrode layer. In addition, the conductivelayer 417 may have a potential which is different from that of the gateelectrode layer 421 a. Alternatively, the potential of the conductivelayer 417 may be GND or 0 V, or the conductive layer 417 may be in afloating state.

Since the thin film transistor is easily broken due to staticelectricity or the like, a protective circuit is preferably providedover the same substrate as the pixel portion or the driver circuitportion. The protective circuit is preferably formed with a non-linearelement including an oxide semiconductor layer. For example, protectivecircuits are provided between the pixel portion and each of a scan-lineinput terminal and a signal-line input terminal. In this embodiment, aplurality of protective circuits are provided so as to prevent breakageof a pixel transistor and the like which can be caused when a surgevoltage due to static electricity or the like is applied to a scan line,a signal line, and a capacitor bus line. Therefore, the protectivecircuit is formed so as to release charge to a common wiring when asurge voltage is applied to the protective circuit. Further, theprotective circuit includes non-linear elements arranged in parallel toeach other with the scan line therebetween. The non-linear elementincludes a two-terminal element such as a diode or a three-terminalelement such as a transistor. For example, the non-linear element can beformed through the same step as the thin film transistor 460 in thepixel portion, and can be made to have the same characteristics as adiode by connecting a gate terminal to a drain terminal of thenon-linear element, for example.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 5

A semiconductor device having a display function (also referred to as adisplay device) can be manufactured using the thin film transistordescribed in any of Embodiments 1, 2, and 3. Moreover, a driver circuitportion and a pixel portion, which each include the thin filmtransistor, can be formed over one substrate, whereby a system-on-panelcan be obtained.

A display device includes a display element. Further, a display mediumwhose contrast is changed by an electric effect, such as a liquidcrystal element (also referred to as a liquid crystal display element)or an electronic ink, can be used.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the “display device” includes the following modules inits category: a module including a flexible printed circuit (FPC) or atape automated bonding (TAB) tape; a module having a TAB tape which isprovided with a printed wiring board at the end thereof; and a modulehaving an integrated circuit (IC) which is directly mounted on a displayelement with a chip on glass (COG) method.

The appearance and a cross section of a liquid crystal display panel,which is an embodiment of a display device, will be described withreference to FIGS. 6A1, 6A2, and 6B. FIGS. 6A1 and 6A2 are each a planview of a panel in which thin film transistors 4010 and 4011 and aliquid crystal element 4013 are sealed between a first substrate 4001and a second substrate 4006 with a sealant 4005 interposed therebetween.FIG. 6B is a cross-sectional view taken along line M-N in FIGS. 6A1 and6A2.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan-line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan-line driver circuit 4004. Therefore, the pixelportion 4002 and the scan-line driver circuit 4004 are sealed togetherwith a liquid crystal layer 4008, by the first substrate 4001, thesealant 4005, and the second substrate 4006. Further, a signal-linedriver circuit 4003 which is formed using a single crystal semiconductoror a polycrystalline semiconductor is mounted in a region different fromthe region surrounded by the sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on the connection method ofthe signal-line driver circuit 4003, and a COG method, a wire bondingmethod, a TAB method, or the like can be used. FIG. 6A1 illustrates anexample in which the signal-line driver circuit 4003 is mounted with aCOG method, and FIG. 6A2 illustrates an example in which the signal-linedriver circuit 4003 is mounted with a TAB method.

Each of the pixel portion 4002 and the scan-line driver circuit 4004which are provided over the first substrate 4001 includes a plurality ofthin film transistors. In FIG. 6B, the thin film transistor 4010included in the pixel portion 4002 and the thin film transistor 4011included in the scan-line driver circuit 4004 are illustrated as anexample. Insulating layers 4041, 4020, and 4021 are provided over thethin film transistors 4010 and 4011.

Any of the highly reliable thin film transistors each including an oxidesemiconductor layer, which are described in any of Embodiments 1, 2, and3, can be used as the thin film transistors 4010 and 4011. In thisembodiment, the thin film transistors 4010 and 4011 are n-channel thinfilm transistors.

A conductive layer 4040 is provided over the insulating layer 4021 so asto overlap with a channel formation region of an oxide semiconductorlayer in the thin film transistor 4011 for the driver circuit. Theconductive layer 4040 is provided so as to overlap with the channelformation region of the oxide semiconductor layer, whereby the amount ofchange over time in threshold voltage of the thin film transistor 4011can be reduced. The conductive layer 4040 has a potential which is thesame as that of the gate electrode of the thin film transistor 4011, andcan function as a second gate electrode layer. In addition, theconductive layer 4040 may have a potential which is different from thatof the gate electrode of the thin film transistor 4011. Alternatively,the potential of the conductive layer 4040 may be GND or 0 V, or theconductive layer 4040 may be in a floating state.

A pixel electrode 4030 included in the liquid crystal element 4013 iselectrically connected to the thin film transistor 4010. A counterelectrode 4031 of the liquid crystal element 4013 is formed over thesecond substrate 4006. The liquid crystal element 4013 corresponds to aregion where the pixel electrode 4030, the counter electrode 4031, andthe liquid crystal layer 4008 overlap with each other. Note that thepixel electrode 4030 and the counter electrode 4031 are provided with aninsulating layer 4032 and an insulating layer 4033 which each functionas an alignment film, respectively.

Note that a light-transmitting substrate such as glass, ceramics, orplastics can be used as the first substrate 4001 and the secondsubstrate 4006. As plastic, a fiberglass-reinforced plastics (FRP)plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylicresin film can be used.

A columnar spacer denoted by reference numeral 4035 is obtained byselective etching of an insulating layer and is provided in order tocontrol the distance (a cell gap) between the pixel electrode 4030 andthe counter electrode 4031. Note that a spherical spacer may be used.The counter electrode 4031 is electrically connected to a commonpotential line formed over the same substrate as the thin filmtransistor 4010. The counter electrode 4031 and the common potentialline can be electrically connected to each other through conductiveparticles arranged between the pair of substrates, using a commonconnection portion. Note that the conductive particles are included inthe sealant 4005.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase is generated within an onlynarrow range of temperature, liquid crystal composition containing achiral agent at 5 wt % or more so as to improve the temperature range isused for the liquid crystal layer 4008. The liquid crystal compositionwhich includes liquid crystal exhibiting a blue phase and a chiral agenthas a short response time of 1 msec or less, has optical isotropy, whichmakes the alignment process unneeded, and has a small viewing angledependence.

An example of the liquid crystal display device is described in which apolarizing plate is provided on the outer surface of the substrate (onthe viewer side) and a coloring layer and an electrode layer used for adisplay element are provided on the inner surface of the substrate inthis order; however, the polarizing plate may be provided on the innersurface of the substrate. The stacked structure of the polarizing plateand the coloring layer is not limited to this embodiment and may be setas appropriate depending on materials of the polarizing plate and thecoloring layer or conditions of manufacturing process.

In the thin film transistor 4011, the insulating layer 4041 is formed incontact with the semiconductor layer including the channel formationregion. The insulating layer 4041 can be formed using a material and amethod which are similar to those of the insulating layer 427 describedin Embodiment 1. In order to reduce the surface roughness of the thinfilm transistor, the thin film transistors are covered with theinsulating layer 4021 which functions as a planarizing insulating layer.Here, a silicon oxide film is formed as the insulating layer 4041 with asputtering method in a manner similar to that of Embodiment 1.

A protective insulating layer 4020 is formed over the insulating layer4041. The protective insulating layer 4020 can be formed using amaterial and a method which are similar to those of the protectiveinsulating layer 428 described in Embodiment 1. Here, a silicon nitridefilm is formed as the protective insulating layer 4020 with a PCVDmethod.

The insulating layer 4021 is formed as the planarizing insulating layer.The insulating layer 4021 can be formed using a heat-resistant organicmaterial such as polyimide, acrylic, benzocyclobutene, polyamide, orepoxy. Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like. Note that the insulating layer 4021 may be formed by stackinga plurality of insulating layers formed using these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include an organic group (e.g.,an alkyl group or an aryl group) or a fluoro group as a substituent. Inaddition, the organic group may include a fluoro group.

The insulating layer 4021 can be formed, depending on the material, witha method such as a sputtering method, an SOG method, a spin coatingmethod, a dipping method, a spray coating method, a droplet dischargemethod (e.g., an ink-jet method, screen printing, or offset printing),or a tool such as a doctor knife, a roll coater, a curtain coater, or aknife coater. The step of baking the insulating layer 4021 serves alsoas the annealing step of the semiconductor layer, whereby the displaydevice can be efficiently manufactured.

The pixel electrode 4030 and the counter electrode 4031 can be formedusing a light-transmitting conductive material such as indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added.

Alternatively, the pixel electrode 4030 and the counter electrode 4031can be formed using a conductive composition containing a conductivemacromolecule (also referred to as a conductive polymer). It ispreferable that the pixel electrode formed using the conductivecomposition have a sheet resistance of 10,000 Ω/square or less and alight transmittance of 70% or more at a wavelength of 550 nm. Further,the resistivity of the conductive macromolecule included in theconductive composition is preferably less than or equal to 0.1 Ω·cm.

As the conductive macromolecule, a so-called π-electron conjugatedconductive polymer can be used. For example, polyaniline or a derivativethereof, polypyrrole or a derivative thereof, polythiophene or aderivative thereof, a copolymer of two or more kinds of them, and thelike can be given.

Further, a variety of signals and a potential are supplied to thesignal-line driver circuit 4003 which is formed separately, thescan-line driver circuit 4004, and the pixel portion 4002 through an FPC4018.

In addition, a connection terminal electrode 4015 is formed from thesame conductive film as the pixel electrode 4030 included in the liquidcrystal element 4013, and a terminal electrode 4016 is formed from thesame conductive film as the source electrode layers and the drainelectrode layers of the thin film transistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductive film4019.

Note that FIGS. 6A1, 6A2, and 6B illustrate an example in which thesignal-line driver circuit 4003 is formed separately and mounted on thefirst substrate 4001; however, this embodiment is not limited to thisstructure. The scan-line driver circuit may be separately formed andthen mounted, or only part of the signal-line driver circuit or part ofthe scan-line driver circuit may be separately formed and then mounted.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 6

In this embodiment are described the appearance and a cross section of alight-emitting display panel (also referred to as a light-emittingpanel), with reference to FIGS. 7A and 7B. FIG. 7A is a plan view of apanel in which a thin film transistor and a light-emitting elementformed over a first substrate are sealed between the first substrate anda second substrate with a sealant. FIG. 7B is a cross-sectional viewtaken along line H-I in FIG. 7A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal-line driver circuits 4503 a and 4503 b, and scan-line drivercircuits 4504 a and 4504 b which are provided over a first substrate4501. In addition, a second substrate 4506 is provided over the pixelportion 4502, the signal line-driver circuits 4503 a and 4503 b, and thescan-line driver circuits 4504 a and 4504 b.

Accordingly, the pixel portion 4502, the signal-line driver circuits4503 a and 4503 b, and the scan-line driver circuits 4504 a and 4504 bare sealed together with a filler 4507, by the first substrate 4501, thesealant 4505, and the second substrate 4506. It is preferable that apanel be packaged (sealed) with a protective film (such as a bondingfilm or an ultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the panel is not exposedto the outside air, in this manner.

Further, each of the pixel portion 4502, the signal-line driver circuits4503 a and 4503 b, and the scan-line driver circuits 4504 a and 4504 bwhich are provided over the first substrate 4501 includes a plurality ofthin film transistors. In FIG. 7B, a thin film transistor 4510 includedin the pixel portion 4502 and a thin film transistor 4509 included inthe signal-line driver circuit 4503 a are illustrated as an example.

Any of the highly reliable thin film transistors each including an oxidesemiconductor layer, which are described in any of Embodiments 1, 2, and3, can be used as the thin film transistors 4509 and 4510. In thisembodiment, the thin film transistors 4509 and 4510 are n-channel thinfilm transistors.

A conductive layer 4540 is provided over an insulating layer 4544 so asto overlap with a channel formation region of an oxide semiconductorlayer in the thin film transistor 4509 for the driver circuit. Theconductive layer 4540 is provided so as to overlap with the channelformation region of the oxide semiconductor layer, whereby the amount ofchange over time in threshold voltage of the thin film transistor 4011before and after the BT test can be reduced. The conductive layer 4540has a potential which is the same as that of the gate electrode of thethin film transistor 4509, and can function as a second gate electrodelayer. In addition, the conductive layer 4540 may have a potential whichis different from that of the gate electrode of the thin film transistor4509. Alternatively, the potential of the conductive layer 4540 may beGND or 0 V, or the conductive layer 4540 may be in a floating state.

In a periphery of the oxide semiconductor layer of the thin filmtransistor 4509, an oxide insulating layer 4542 which covers aperipheral portion (including a side surface) of the oxide semiconductorlayer is formed.

Further, the thin film transistor 4510 is electrically connected to afirst electrode 4517 through a connection electrode layer 4548. Further,the oxide insulating layer 4542 which covers a peripheral portion(including a side surface) of the oxide semiconductor layer of the thinfilm transistor 4510 is formed.

The oxide insulating layer 4542 can be formed using a material and amethod which is similar to that of the oxide insulating layer 426described in Embodiment 1. In addition, the insulating layer 4544 whichcovers the oxide insulating layer 4542 is formed. The insulating layer4544 can be formed using a material and a method which are similar tothose of the protective insulating layer 428 described in Embodiment 1.

A color filter layer 4545 is formed over the thin film transistor 4510so as to overlap with a light-emitting region of a light-emittingelement 4511.

Further, in order to reduce the surface roughness of the color filterlayer 4545, the color filter layer 4545 is covered with an overcoatlayer 4543 which functions as a planarizing insulating film.

Further, an insulating layer 4546 is formed over the overcoat layer4543. The insulating layer 4546 can be formed using a material and amethod which are similar to those of the protective insulating layer 428described in Embodiment 1.

The first electrode 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a sourceelectrode layer or a drain electrode layer of the thin film transistor4510. Note that the structure of the light-emitting element 4511 is notlimited to a stacked structure of the first electrode 4517, anelectroluminescent layer 4512, and a second electrode 4513. Thestructure of the light-emitting element 4511 can be changed asappropriate depending on, for example, the direction in which light isextracted from the light-emitting element 4511.

A partition wall 4520 can be formed using an organic resin film, aninorganic insulating film, or organic polysiloxane. It is preferablethat the partition wall 4520 be formed to have an opening over the firstelectrode 4517 so that a sidewall is formed as an inclined surface withcurvature. Such an opening can be easily formed using a photosensitiveresin material.

The electroluminescent layer 4512 is not limited to a single layer andmay be formed using a plurality of layers stacked.

A protective film may be formed over a second electrode 4513 and thepartition wall 4520 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4511. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

Further, a variety of signals and a potential are supplied to thesignal-line driver circuits 4503 a and 4503 b, the scan-line drivercircuits 4504 a and 4504 b, and the pixel portion 4502 through FPCs 4518a and 4518 b.

In addition, a connection terminal electrode 4515 is formed from thesame conductive film as the first electrode 4517 included in thelight-emitting element 4511, and a terminal electrode 4516 is formedfrom the same conductive film as a source electrode layer and a drainelectrode layer of the thin film transistor 4509.

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a through an anisotropic conductivefilm 4519.

As the second substrate located in the direction in which light isextracted from the light-emitting element 4511 needs to have alight-transmitting property. In that case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used for the second substrate.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon. For example, polyvinyl chloride (PVC), acrylic, polyimide, anepoxy resin, a silicone resin, polyvinyl butyral (PVB), or ethylenevinyl acetate (EVA) can be used.

If necessary, an optical film such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter may be provided as appropriate for a light-emitting surfaceof the light-emitting element. Further, a polarizing plate or acircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

As the signal-line driver circuits 4503 a and 4503 b and the scan-linedriver circuits 4504 a and 4504 b, a driver circuit formed using asingle crystal semiconductor or a polycrystalline semiconductor may bemounted. Alternatively, only the signal line-driver circuits or partthereof, or only the scan-line driver circuits or part thereof may beseparately formed and mounted. This embodiment is not limited to thestructure illustrated in FIGS. 7A and 7B.

Through the steps described above, a highly reliable light-emittingdisplay device (display panel) can be manufactured.

Note that this embodiment can be freely combined with any of the otherembodiments.

Embodiment 7

A display device disclosed in this specification can be applied to avariety of electronic devices (including an amusement machine). Examplesof electronic devices are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a cellular phone (also referred to as a mobilephone or a cellular phone set), a portable game console, a portableinformation terminal, an audio reproducing device, a large-sized gamemachine such as a pachinko machine, and the like.

FIG. 8A illustrates an example of a cellular phone. The cellular phone1100 is provided with a display portion 1102 incorporated in a housing1101, an operation button 1103, an external connection port 1104, aspeaker 1105, a microphone 1106, and the like.

When the display portion 1102 of the cellular phone 1100 is touched witha finger or the like, data can be inputted into the cellular phone 1100.Further, operations such as making calls, composing mails, or the likecan be performed by touching the display portion 1102 with a finger orthe like.

There are mainly three screen modes of the display portion 1102. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode, which is a combination of the twomodes, i.e. a combination of the display mode and the input mode.

For example, in the case where a call is made or a mail is composed, atext input mode mainly for inputting text is selected for the displayportion 1102 so that text displayed on a screen can be inputted. In thatcase, it is preferable to display a keyboard or number buttons on almostall area of the screen of the display portion 1102.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside thecellular phone 1100, display on the screen of the display portion 1102can be automatically switched by determining the direction of thecellular phone 1100 (whether the cellular phone 1100 is placedhorizontally or vertically for a landscape mode or a portrait mode).

The screen modes are switched by touching the display portion 1102 oroperating the operation button 1103 of the housing 1101. Alternatively,the screen modes may be switched depending on the kind of the imagedisplayed on the display portion 1102. For example, when a signal of animage displayed on the display portion is a signal of moving image data,the screen mode is switched to the display mode. When the signal is asignal of text data, the screen mode is switched to the input mode.

Further, in the input mode, when input by touching the display portion1102 is not performed for a certain period while a signal detected bythe optical sensor in the display portion 1102 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 1102 can also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenwhen the display portion 1102 is touched with a palm or a finger,whereby personal identification can be performed. Further, by providinga backlight or a sensing light source which emits a near-infrared lightin the display portion, an image of a finger vein, a palm vein, or thelike can be taken.

FIG. 8B also illustrates an example of a cellular phone. A portableinformation terminal whose example is illustrated in FIG. 8B can have aplurality of functions. For example, in addition to a telephonefunction, such a portable information terminal can have a function ofprocessing a variety of data by incorporating a computer.

The portable information terminal illustrated in FIG. 8B includes twohousings, a housing 1800 and a housing 1801. The housing 1800 includes adisplay panel 1802, a speaker 1803, a microphone 1804, a pointing device1806, a camera 1807, an external connection terminal 1808, and the like.The housing 1801 includes a keyboard 1810, an external memory slot 1811,and the like. In addition, an antenna is incorporated in the housing1801.

The display panel 1802 is provided with a touch panel. A plurality ofoperation keys 1805 which is displayed as images is illustrated bydashed lines in FIG. 8B.

Further, in addition to the above structure, a contactless IC chip, asmall memory device, or the like may be incorporated.

The display device can be used for the display panel 1802 and thedirection of display is changed appropriately depending on anapplication mode. Further, the display device is provided with thecamera 1807 on the same surface as the display panel 1802, and thus itcan be used as a videophone. The speaker 1803 and the microphone 1804can be used for videophone calls, recording, and playing sound, and thelike as well as voice calls. Moreover, the housings 1800 and 1801 in astate where they are developed as illustrated in FIG. 8B can shift sothat one is lapped over the other by sliding; therefore, the size of theportable information terminal can be reduced, which makes the portableinformation terminal suitable for being carried.

The external connection terminal 1808 can be connected to various typesof cables such as a charging cable and a USB cable, and charging anddata communication with a personal computer are possible. Moreover, astorage medium can be inserted into the external memory slot 1811 sothat a larger amount of data can be stored and can be moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 9A illustrates an example of a television set 9600. In thetelevision set 9600, a display portion 9603 is incorporated in a housing9601. The display portion 9603 can display images. Here, the housing9601 is supported by a stand 9605.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled with operation keys 9609 of the remote controller 9610so that an image displayed on the display portion 9603 can becontrolled. Furthermore, the remote controller 9610 may be provided witha display portion 9607 for displaying data outputted from the remotecontroller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television set 9600 isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 9B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated in a housing 9701. The display portion 9703 can display avariety of images. For example, the display portion 9703 can displaydata of an image taken with a digital camera or the like and function asa normal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (e.g., a USB terminal), anexternal memory slot, and the like. Although these components may beprovided on the surface on which the display portion is provided, it ispreferable to provide them on the side surface or the back surface forthe design of the digital photo frame 9700. For example, a memorystoring data of an image taken by a digital camera is inserted in theexternal memory slot of the digital photo frame, and the image data canbe transferred and displayed on the display portion 9703.

The digital photo frame 9700 may be configured to transmit and receivedata wirelessly. The structure may be employed in which desired imagedata is transferred wirelessly and displayed.

FIG. 10 is a portable game machine and includes two housings, a housing9881 and a housing 9891, which are connected with a joint portion 9893so that the portable game machine can be opened or folded. A displayportion 9882 and a display portion 9883 are incorporated in the housing9881 and the housing 9891, respectively.

Moreover, the portable game machine illustrated in FIG. 10 is providedwith a speaker portion 9884, a recording medium insertion portion 9886,an LED lamp 9890, input means (operation keys 9885, a connectionterminal 9887, a sensor 9888 (having a function of measuring force,displacement, position, velocity, acceleration, angular velocity,rotation number, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radial ray, flow rate, humidity, gradient,vibration, odor, or infrared ray), and a microphone 9889, and the like.It is needless to say that the structure of the portable amusementmachine is not limited to the above and other structures provided withat least a thin film transistor disclosed in this specification can beemployed. The portable amusement machine may include other accessoryequipment as appropriate. The portable game machine illustrated in FIG.10 has a function of reading a program or data stored in a recordingmedium to display it on the display portion, and a function of sharinginformation with another portable game machine by wirelesscommunication. Note that functions of the portable game machineillustrated in FIG. 10 are not limited to those described above and theportable game machine can have a variety of functions.

As described above, the display device described in other embodimentscan be arranged in display panels of a variety of electronic appliancessuch as the above ones.

Note that this embodiment can be freely combined with any of the otherembodiments.

The present application is based on Japanese Patent Application serialNo. 2009-204801 filed with the Japan Patent Office on Sep. 4, 2009, theentire contents of which are hereby incorporated by reference.

The invention claimed is:
 1. A method for manufacturing a semiconductordevice, comprising: forming an oxide semiconductor layer; performing afirst heat treatment at a temperature equal to or higher than 400° C.;forming an oxide insulating layer over and in contact with the oxidesemiconductor layer; performing a second heat treatment after formingthe oxide insulating layer at a temperature equal to or higher than 200°C. and equal to or lower than 400° C.; etching part of the oxideinsulating layer so that the etched oxide insulating layer covers anedge portion of the oxide semiconductor layer; and forming a sourceelectrode and a drain electrode over the oxide insulating layer, whereinthe source electrode and the drain electrode are in direct contact withthe oxide semiconductor layer.
 2. The method according to claim 1,wherein the first heat treatment is performed to dehydrate ordehydrogenate the oxide semiconductor layer.
 3. The method according toclaim 1, wherein oxygen is supplied from the oxide insulating layer tothe oxide semiconductor layer by the second heat treatment.
 4. Themethod according to claim 1, further comprising: forming an insulatinglayer over the source electrode and the drain electrode.
 5. The methodaccording to claim 4, further comprising: forming an electrode over theinsulating layer.
 6. The method according to claim 5, furthercomprising: forming an EL layer over and in contact with the electrode,wherein the electrode is electrically connected with one of the sourceelectrode and the drain electrode.
 7. The method according to claim 1,wherein the edge portion of the oxide semiconductor layer is in anoxygen-excess state by oxygen supplied from the oxide insulating layer.8. The method according to claim 1, wherein the oxide semiconductorlayer comprises indium.
 9. A method for manufacturing a semiconductordevice, comprising: forming an oxide semiconductor layer; performing afirst heat treatment to the oxide semiconductor layer; forming aninsulating layer over the oxide semiconductor layer after performing thefirst heat treatment; performing a second heat treatment on theinsulating layer and the oxide semiconductor layer; etching part of theinsulating layer so that the etched insulating layer covers an edgeportion of the oxide semiconductor layer; and forming an electrode layerover the insulating layer, wherein the maximum temperature of the secondheat treatment is equal to or higher than 200° C. and equal to or lowerthan 400° C., wherein the maximum temperature of the first heattreatment is equal to or higher than the maximum temperature of thesecond heat treatment, and wherein the electrode layer is in directcontact with the oxide semiconductor layer.
 10. The method according toclaim 9, wherein the oxide semiconductor layer comprises indium.
 11. Themethod according to claim 9, wherein concentration of hydrogen in theoxide semiconductor layer is reduced and oxygen vacancies are increasedby the first heat treatment.